The Halloween Party
by ANGSWIN
Summary: When John throws a Halloween Party for Rosie and her friends, Sherlock relieves his boredom by observing an unusual family attending it...and comes to an astonishing deduction! Written by request for the 2017 BBC Sherlock Trick-or-Treat Exchange


_This story was written for **pendingFriendship** over on AO3 for the 2017 BBC Sherlock Trick-or-Treat Exchange._

 _She requested a Sherlock/ Harry Potter Halloween crossover fic with Muggleborns and magic where Sherlock interacts with children and questions his own deductions...and I was happy to oblige! :)_

* * *

 **The Halloween Party**

"I really don't see why my presence is required tonight, John," Sherlock said petulantly for the umpteenth time. "Both Molly and Ms. Hudson will be there, so you will have plenty of help. You don't need me."

"You're the godfather, Sherlock." John replied patiently. "You are supposed to be there for all of Rosie's firsts and this is her first Halloween party. As I've already told you several times, I'm going to need all of the adults that I can get to supervise and run the games because there are going to be lots of children there. Since I don't know many responsible adults, I am going to have to make do with you!" He paused for a moment to grin affectionately at his friend before he continued. "That's including some new friends that she just made, by the way. You know that nice park that Rosie likes between Camden Town and Grimmauld Place? Well, we lucked out and it was completely empty yesterday except for this very nice couple and their kids. The youngest was a little girl named Lily who was about the same age as Rosie and they hit it off right from the start. Even the two older boys played with her, too. The parents seemed really interesting, as well, so I invited them all to the party. I think that even you might like the father. He is some sort of special investigator. He was a bit vague about the details, though. However, he was quite distracted by the children. His wife was quite the redheaded firecracker and some kind of sports journalist, I believe.

"A special investigator and a journalist…sounds like a match made in Heaven," Sherlock scoffed before trying one more time to get out of party duty.

John wasn't having any of it, however. "Six o'clock sharp, Sherlock…and wear a costume! Mrs. Hudson, Molly, and I are all going Victorian…" His words trailed off at the sight of the expression of disdain on Sherlock's face. "What?" he asked defensively. "They are helping me to host, so we thought it would be nice if we all matched."

Sherlock made a small sound of derision. A children's Halloween party was not very high on the list of ways he wanted to spend his evening…and to have to wear a costume on top of that? He would rather spend the evening watching Mycroft eat cake. However, he knew that he would do it, anyway. _The things that I do for John and that little girl_ , he thought before changing the subject. "What kind of costume is Rosie going to wear?

"She is going to be a witch, of course." John answered.

* * *

A family of four approached the door of the Watson home before the mother paused at the stoop to whisper fervently to her husband, "Are you sure about this, Harry?"

He patted her on the shoulder reassuringly. "Yes, I am. The kids need more practice in the Muggle world, Ginny. You know I want them to know more about my heritage and we can't let Hermione have all of the responsibility of teaching them that. They are our children, after all! Plus, a Halloween party is the perfect place for them to practice being Muggles. If anything _strange_ or _unusual_ happens, it will be lost in the crowd or blamed on the holiday. The absolute worst case scenario will be that we will have to perform a few selective _Obliviates_ before we leave – which I am licensed to do. Therefore, don't worry. It will be fine."

"That's what you always say," Ginny sighed as she straightened her witch's robes and hat and smiled down at Albus and Lily, who were dressed up as younger versions of their parents – complete with school robes. Even though wizards and witches do celebrate Halloween, and the more traditional Samhain, they don't dress up in costumes for it. Therefore, when Hermione mentioned that they would need costumes for the party, they did the best that they could at the last minute. Ginny thought her children looked adorable at any rate – especially Albus who had even drawn a little lightning-shaped scar on his forehead to match his father's famous one. Ginny was momentarily sad that James had not wanted to accompany them, but her eldest son had decided to stay with Teddy and Andromeda instead. He loved hanging out with the older boy and had not wanted to go to what he called a _baby party_ with his younger siblings, anyway. She shook her head and refocused on Albus and Lily. "At least you two will have some interesting stories to tell your grandfather," she said to them while thinking about how much Arthur Weasley loved anything Muggle related. Harry, meanwhile, showed the children how to use the knocker on the door to announce their arrival.

* * *

Sherlock scanned the room looking for _anything_ interesting. He had quickly become bored with the festivities and the rest of the people soon after his arrival. It was a children's party, after all. He had already endured the goodnatured laughter of both John and Molly, as well as his landlady, when he had shown up in his deerstalker, ulster, and a pipe to represent what he thought he would have looked like as himself in the Victorian era. He felt that John was being a bit ridiculous about the whole thing, however – especially since he was attired in a similar style and even sported a mustache reminiscent of the completely unsuitable one that he had grown after Sherlock's _death._

John, envisioning the disasters that would occur if he put Sherlock in charge of any of the games, stationed him at the adult refreshment table instead and told him to watch out for saboteurs and/or sneaky kids. Sherlock felt that this assignment was a waste of his talents – especially when he watched Molly help the kids reach into buckets to touch yucky "body parts" such as eyeballs (frozen grapes) and brains (cold spaghetti). Even Mrs. Hudson had a more interesting job than he did. She was helping the kids to draw chalk and crayon outlines of themselves, so that they could decorate them. In fact, Sherlock probably would have left immediately, if a pretty little witch had not run up and flung herself into his arms with an excited squeal, "Unk Shuluk! Imma witch! Imma going to turn you inna frog!" He pretended to be scared for a moment until she laughed and gave him a kiss on his nose before running back off again to play. He knew then that he would stay just for her. He sighed and continued scanning the room until he noticed the arrival of another family being greeted by John and Rosie before they were brought over briefly to meet him and to get a cup of punch.

Right away, they seemed _different_. For one, their costumes were…unique. The adults were dressed as a witch and wizard, but more like Victorian-style depictions, rather than modern costumes like the one his goddaughter and several other children and adults wore. Plus, the man – a _special investigator_ , John had said – carried a broom with him. However, it was unlike any kind of broom that Sherlock had ever seen before. This one was sleek and shiny and definitely wasn't used to sweep floors! In fact, as unlikely as it seemed, it had a quality that simply screamed _aerodynamic_ and Sherlock could see the words, _Firebolt X_ , printed on the side. He had never seen anything like it and his fingers practically itched to examine it. He tried to repress this impulse, however, since John seemed to disapprove of such actions. Ordinarily, he wouldn't care, but he wanted for John and Rosie's party to go smoothly. Therefore, he forced his attention away from the broom and transferred it to the children, instead. He noticed that they were dressed as a cross between typical uniformed schoolchildren and an old fashioned witch and wizard – just like their parents. In fact, once he saw the matching scars on the males and the matching hair color and styles on the females of the family, he deduced that the younger ones were dressed up to represent their parents as children. Is that the kind of outfit that they would have worn to school? It was all quite…interesting…and he decided to put on his deadpan surveillance face and keep an eye on them – if only to relieve the crushing boredom that he otherwise felt.

His attention was rewarded when the two children ran off to play with little Rosie. The man, Harry, ( _leader, authority, comfortable in his surroundings_ ) had walked over to the television set where a kids' Halloween movie was playing and was gesturing to it and intensely explaining something about electronics to his wife, Ginny, ( _capable, confident, a bit spoiled – only child or only girl?, distinctly uncomfortable with these particular surroundings_ ). After just a couple of moments, they were approached by another man whom Sherlock barely remembered being introduced to earlier as a father of a girl with whom Rosie attended nursery school. Sherlock moved a little closer to hear the exchange.

"Is that the new _Firebolt X_?" he heard the man ask in an excited but quiet tone that was designed not to be overheard. Harry, who clearly had not expected the question, spun around, and Sherlock watched as he dropped into a defensive position – almost as if he was expecting to be attacked. _A soldier…that's intriguing!_ The first man gasped, "You're Harry Potter!" He reached out to shake hands as Harry stood up, sheepishly. "And Ginny Weasley!" he exclaimed almost as excited to see the wife. "I'm a big fan of you both, of course." _They are famous?_ Sherlock wondered as he surreptitiously took out his phone to Google them, but didn't get any hits. He was just thinking about hacking into Mycroft's database from his laptop when he got home, when he realized that the man was still talking. "I know that you don't remember me from Hogwarts. I was several years behind you, and actually left for a couple of years during the…well…you know…unpleasantness." _Unpleasantness – what does that mean?_ "However, I think you probably know my brother, Justin…Justin Finch-Fletchley?" At the look of dawning comprehension on the couple's face, the newcomer had to laugh. "Yeah, I know…he was a bit of a prat back then, but he grew out of it - mostly. I'm Daniel, by the way. I was a beater for the Ravenclaw team my last two years after I came back, but you guys had already graduated by then. I didn't mean to surprise you, but I just saw your broom and I couldn't resist. I have a _Nimbus Infinity_ , which is a bloody good model, but not quite up to the new _Firebolt_ standards."

The conversation then apparently turned into a lively discussion where all three sang the praises about the merits of their various broomsticks (apparently the woman preferred something called the _Clean Sweep Cyclone_ ), leaving Sherlock completely bewildered. _They actually sound as if they fly…and play a game…on those things!_ He remembered that the woman was apparently some kind of sports journalist…and then immediately started wondering if someone had indeed already sabotaged the adult punch bowl and had spiked it even further than that bit of rum that he had seen Mrs. Hudson mix in. In his boredom, he had consumed several cups of it, but had felt no ill effects…until now… apparently. Before he could contemplate this further, however, he was distracted when he felt a tug on his ulster. He looked down to see that the son of the couple that he had just been watching was standing next to him.

"Excuse me, sir," the little boy said. _Probably around six years of age - about two years older than his little sister_ _and Rosie, very polite and mature for his age, probably a bit of a loner and plays mostly with his siblings,_ concluded Sherlock _._ "Where are Lily and Rosie? We were going to bob for apples, but now I can't find them. Rosie's father told me to ask you." Sherlock, quite surprised at being addressed by a child that was not his goddaughter, just looked at him for a moment and observed that he had similar features to both of his parents. He then looked up in John's direction where he was helping some children at the apple tub. John met his eye and gave him a merry wave and a smirk. _Probably thinking that it was hilarious to force me to talk a child whom I don't know._ Sherlock just shook his head at the man's antics. However, he humored his friend and the boy, and looked around – using his keen eye for details to notice a couple of little feet sticking out from underneath the children's refreshment table.

"I think that you will find your sister and her friend under the table over there," he said seriously and pointed the way.

"Thank you, sir. I'm Albus. I'm dressed up just like my father. What are you supposed to be?"

 _Albus? What kind of name is that?_ thought Sherlock before he realized that he felt a bit of kinship with this serious little boy with a name as equally unusual as his own. Therefore, he squatted down next to him and answered with a little less bite than he probably would have normally. "I'm a consulting detective," he said.

"What's that?" Albus asked curiously.

"I solve mysteries and catch bad guys," Sherlock explained, miraculously remembering that he was conversing with a small child and, therefore, didn't launch into his normally verbose description of his occupation.

"Oh!" Albus exclaimed, his face suddenly alight with understanding. "Just like my father. He catches bad wizards, too!" With those words, and a quick goodbye wave, he was off to join the girls, leaving Sherlock even more contemplative than before. Just then, out of the corner of his eye, he glimpsed what appeared to be a couple of orange cupcakes floating through the air, but they had disappeared by the time that he turned his head to look at them fully. A moment later, the little redheaded girl and his Rosie walked by sharing an orange cupcake - and followed by young Albus who was eating one, as well. _Could it just be a coincidence? The universe is rarely so lazy!_ _What is going on tonight?_ His mind briefly considered airborne mind-control drugs such as the ones that he and John had encountered on the Baskerville case. Many aspects about the happenings at the party reminded him of that night on the moor. He realized, however, that everyone would be acting strangely, if that was the case - and not just him and a handful of the guests. While he pondered this idea, his attention was drawn back to the conversation of the apparently broom-obsessed adults.

"Well, I'm Muggleborn, of course. However, my wife is a teacher at Lucy and Rosie's nursery school. That's why I am on party parent duty tonight. She said that she has been dealing with little kids all week and now it's my turn!" The man named Daniel paused to laugh before he continued. "Anyway, she's a Muggle, through and through, absolutely no magic whatsoever - so we weren't sure about about little Luce for the first couple of years. However, it now looks like she will be attending Hogwarts, as well. She's had several little bursts of accidental magic in the past couple of months. Only last week, she blew up the dog. He was as big as the hedge before I found them! Fortunately, they were in the back garden and I was able to use a deflating charm before the neighbors noticed. The dog wasn't very happy about the whole situation, however."

Harry snickered, "I once did the same thing to my aunt! However, it was more like accidental hormonal anger magic back then…since I was thirteen at the time!" The little group laughed.

 _Muggles, magic, charms?_ Sherlock's head was whirling. _It just couldn't be true, could it?_

Just then, he was distracted from his thoughts, yet again. Their children, with the addition of Rosie, ran by. However, the little ginger haired girl stopped for hugs and kisses from her mum while the little brunette ran right up to Daniel, who was apparently her father.

"Daddy, daddy," little Lucy cried out loudly. "Lily is goin' go to witch school, too!"

"Is that so?" Daniel answered warmly. "Maybe she will be an eagle in Ravenclaw with you!"

"No, Imma goin' be a lion in Grif'dor like Mummy and Daddy!" little Lily exclaimed with a roar while Lucy pretended to fly around like an eagle. Then Rosie joined in and took turns being both an eagle and a lion, while Albus stood seriously by his father's side and just watched while the younger girls acted silly. John wandered over to watch the show, as well, and the adults all laughed at the antics of their children. Sherlock just stood back and observed them all - while he desperately tried to come to some conclusions that actually made sense!

Soon it was time for the party to be over. Prizes were collected, friends were hugged, thanks were given, and mobile numbers were exchanged between parents. Basically the children were happy, but tired. However, the adults were exhausted and more than ready to get home, put the little ones to bed, and have some quiet time for themselves. Therefore, the Watson home quickly emptied, as all of the guests went their separate ways.

* * *

"Well, that went better than I expected." Ginny told her husband as they walked around a blind corner and each gathered up a child in order to apparate home.

"I told you that it would be fine," Harry replied smugly once they had landed in the foyer of Grimmauld Place. "Halloween is a great time to mingle with the Muggles because you can talk about witches and spells and broomsticks…and nobody thinks that it is unusual, at all! I didn't even have to _Obliviate_ anyone! I must admit that I was beginning to worry about that Sherlock fellow, though. There was a couple of times that I thought that he might have been listening to our conversation with Daniel. However, I must have been mistaken because the man's face never changed expression, not once. Therefore, I just think that he was really _really_ bored!"

Ginny just laughed. "I think that is just the normal state of being for an unmarried and childless godfather at a children's party," she retorted. Harry had to admit that her statement was probably true.

* * *

For his part, Sherlock smiled gently at the very tired little witch who came to give him a sleepy goodnight kiss before she was taken up the stairs by her father and put to bed. The detective then utilized that distraction to make his escape back to Baker Street. Once there, he collapsed into his favorite chair, steepled his fingers against his chin, and immediately slipped into his mind palace where he contemplated everything odd or unusual that he had heard and seen during the party: _Victorian witches and wizards, flying brooms, bad wizard catchers, Hogwarts, accidental magic, witch school, the "unpleasantness", deflating charms, floating cupcakes…_

The detective had always followed the philosophy that "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Therefore, even though it was painful for him to admit it, Sherlock was forced to deduce one thing…He had failed in his duty. At some point, someone must have been able to sneak past him in order to spike the punch bowl, after all! It was the only _reasonable_ explanation that could account for all of the strangeness of the evening. No matter what, in the end, he was a still a man of science! Therefore, he knew that something like magic couldn't possibly exist. Could it?


End file.
